You can't normally purchase on a non-connected PC, but you can use the scenery and activate it, using the offline activation method.

If you have FS9 or FSX on a computer that has internet access, you can purchase the scenery from there, without activating it, by using the "Buy on Web" option. This will just launch a browser, so you'll get an email receipt with your serial number, that you can activate on the non-connected PC, with the offline activation.

Or, if you don't have FS9 or FSX on the connected maching, you can purchase from HERE, which is the same address that would have been accessed by the browser, when launched from FS9.